Both the tension developed by a patient's muscle [34] and the duration of the muscle contraction [2] are factors that lead to respiratory muscle fatigue; these two factors can be expressed by indices such as the tension-time index [3] and the pressure-time product [10, 20, 32, 35]. Bellemare and Grassino [3] showed a direct inverse relationship exists between the time of endurance of a fatiguing diaphragm contraction and the rate of decay of the ratio of the high to low spectral components (H/L) of the electrical activity EAdi of the patient's diaphragm, indicating that these two values are indicative of progressive failure to sustain load. The force exerted by the muscle has been shown to be directly related to the rate of decay of the power spectrum center frequency or the rate of decay of the above mentioned ratio H/L, and the level at which this power spectrum center frequency or ratio H/L plateaus [16, 21, 28]. Such shifts in the power spectrum reflect a reduction in the muscle action potential conduction velocity [28, 38, 39], and constitute an early indication that, at the cellular level, these breathing patterns cannot be maintained indefinitely [3].
Hyperinflation, which impairs the length-tension relationship of the respiratory muscles, i.e. the transformation of the neural activation into a mechanical output or pressure, reduces the capacity of the respiratory muscles to generate pressure (neuromechanical uncoupling), unless the electrical activity EAdi of the patient's diaphragm is increased. Studies have shown that when the inspiratory pressure, flow and duty cycle remain constant, increases in end-expiratory lung volume (EELV) promote reductions in endurance time [33, 44] and sustainable pressure [11]. In an animal model, Tzelepi's et al [44] proposed that, under these conditions, diaphragm shortening would require greater excitation to generate a given sub-maximum tension, and that this increased excitation might account for the greater contractile muscle fatigability observed at shorter muscle length.
The level of partial ventilatory assist, with the aim to ensure adequate pulmonary ventilation while preserving inspiratory muscle function, is generally set on an empirical basis in the clinical setting.
It has been proposed that an optimal level of partial ventilatory assist could be determined from the lowest stable breathing frequency fB achieved, i.e. without bradypnea or apnea. In patients, this corresponded to 16.4 bpm (breaths per minute) and was associated with a tidal volume VT of 11.8 ml/kg. However, mechanical lung modeling in that study demonstrated that such a level of support actually resulted in a total unloading of the respiratory muscles.
Others have defined an optimal level of partial ventilatory assist as that which produces the lowest swings of transdiaphragmatic pressure Pdi and found that this condition was associated with a breathing frequency fB of 19.7 bpm and a tidal volume VT of 11.7 ml/kg. The transdiaphragmatic pressure Pdl in the latter study was used as a marker of inspiratory effort.
Jubran et al [20] defined an upper bound inspiratory pressure-time product lower than 125 cm H2O·s/min as a desirable level of inspiratory effort to be achieved during partial ventilatory assist. Although arbitrarily determined, this threshold was justified by the fact that it corresponded to a tension-time index TTdi well below that considered to indicate impeding inspiratory muscle fatigue. The study found a high variability in pressure-time products between patients and demonstrated that a breathing frequency fB<30 bpm and a tidal volume VT of 0.6 L were better determinants of an optimal level of inspiratory effort during partial ventilatory assist. Based on these breathing pattern findings, it is likely that the level of respiratory muscle unloading provided by this method of optimizing partial ventilatory assist was lower than that of the above discussed studies.
Brochard et al [8] defined an optimal partial ventilatory assist level as the lowest level of ventilatory assist, which when implemented, maintained the highest level of diaphragmatic electrical activation without the occurrence of fatigue as evaluated via power spectrum analysis of the electrical activity EAdi Of the patient's diaphragm. Interestingly, such levels of partial ventilatory assist were associated with a breathing frequency fB of 20-27 bpm and a tidal volume VT of 8.0 ml/kg, these values being similar to those later reported by Jubran et al [20].